Reader/Writer with Semaphores: Take 2+

SharedLock::AcquireRead() {

rblock.P();

if (first reader)

wsem.P();

rblock.V();

}

SharedLock::ReleaseRead() {

if (last reader)

wsem.V();

}

SharedLock::AcquireWrite() {

if (first writer)

rblock.P();

wsem.P();

}

SharedLock::ReleaseWrite() {

wsem.V();

if (last writer)

rblock.V();

}

The rblock prevents readers from entering while writers are waiting.